1. Field of the Invention
The present invention relates to a rotation angle detecting device that detects a rotation angle of a rotating body.
2. Description of Related Art
Brushless motors used in an electric power steering system and the like are controlled by supplying a current to a stator winding in accordance with a rotation angle of a rotor. In order to detect the rotation angle of the rotor, for example, a rotation angle detecting device shown in FIG. 5 is known. The rotation angle detecting device includes a rotor 102 including a magnet having two magnetic poles N, S, and two magnetic sensors 111, 112 that are disposed at an interval of an electrical angle of 90 degrees around a rotation center axis of the rotor 102. The magnetic sensors 111, 112 output their respective sinusoidal signals having a phase difference of 90 degrees therebetween. The rotation angle detecting device detects a rotation angle of the rotor 102 on the basis of the two sinusoidal signals.
A direction indicated by an arrow shown in FIG. 5 is a forward rotation direction of the rotor 102. When the rotor 102 rotates in the forward direction, the rotation angle of the rotor 102 increases. When the rotor 102 rotates in a reverse direction, the rotation angle of the rotor 102 decreases. As shown in FIG. 6, sinusoidal signals having a phase difference of 90 degrees therebetween are output from the magnetic sensors 111, 112, respectively. When an output signal V1 (V1=φ1·sin θ) is output from one magnetic sensor 111 with respect to a rotation angle θ of the rotor 102, an output signal V2 (V2=φ2·sin(θ+90)=φ2·cos θ) is output from the other magnetic sensor 112. Here, each of φ1 and φ2 denotes an amplitude.
When the amplitudes (φ1, φ2 are regarded as the same value φ or when the signals V1, V2 are normalized so that each of the amplitudes is a predetermined specified value φ, the one output signal V1 is expressed by V1=φ·sin θ and the other output signal V2 is expressed by V2=φ·cos θ. Further, when φ is 1 (φ=1), the one output signal V1 is expressed by V1=sin θ, and the other output signal V2 is expressed by V2=cos θ. Accordingly, in order to simplify the description, the output signals V1, V2 of the respective magnetic sensors 111, 112 are expressed by V1=sin θ and V2=sin(θ+90)=cos θ, respectively.
The rotation angle θ of the rotor can be obtained on the basis of, for example, the following Expression (1) using the output signals V1, V2.
                                                        θ              =                            ⁢                                                tan                                      -                    1                                                  ⁡                                  (                                      sin                    ⁢                                                                                  ⁢                                          θ                      /                      cos                                        ⁢                                                                                  ⁢                    θ                                    )                                                                                                        =                            ⁢                                                tan                                      -                    1                                                  ⁡                                  (                                      sin                    ⁢                                                                                  ⁢                                          θ                      /                                              cos                        ⁡                                                  (                                                      θ                            +                            90                                                    )                                                                                                      )                                                                                                        =                            ⁢                                                tan                                      -                    1                                                  ⁡                                  (                                                            V                      1                                        /                                          V                      2                                                        )                                                                                        (        1        )            
Japanese Patent Application Publication No. 2013-61346 (JP 2013-61346 A), Japanese Patent Application Publication No. 2010-101746 (JP 2010-101746 A), and Japanese Patent Application Publication No. 2007-322197 (JP 2007-322197 A) describe examples of the related art.
In the above-described rotation angle detecting device in the related art, when there is a failure in at least one of the magnetic sensors 111, 112, the rotation angle θ of the rotor 102 cannot be detected. Therefore, it is determined whether the magnetic sensors 111, 112 are normal on the basis of the following Expression (2), using the relation of sin2 θ+cos2 θ=1 (relation of V12+V22=1).lower limit≦V12+V22≦upper limit  (2)
The lower limit is set to, for example, 0.9, and the upper limit is set to, for example, 1.1. When Expression (2) is satisfied, it is determined that the magnetic sensors 111, 112 are normal. When Expression (2) is not satisfied, it is determined that there is a failure in at least one of the magnetic sensors 111, 112.
In a case where the two magnetic sensors are disposed at an interval of an electrical angle other than 90 degrees, if the angular interval is not an interval of an electrical angle of 180 degrees, it is possible to detect the rotation angle of the rotating body on the basis of the output signals of the two magnetic sensors. In this case, it is not possible to determine whether the magnetic sensors are normal using Expression (2).